It is known in the art of silicon capacitive microsensors (SCM's) and, more particularly, accelerometers, that such sensors (or transducers) include a silicon capacitive sensing element which senses acceleration forces based on a change in capacitance. Such a sensing element may typically comprise three layers of conductive silicon (i.e., silicon which is doped to be conductive) having glass dielectric layers between each pair of silicon layers. The middle silicon layer consists of a proof mass suspended along one side by one or more silicon hinges which are within and connected to a slightly thicker support layer (or framework) of silicon between the glass layers. There may typically be two metal electrodes, one on the surface of each of the glass dielectric layers which face the silicon proof mass. Each electrode is electrically connected to its corresponding outer silicon layer. A first capacitor exists between one surface of the proof mass and one electrode and a second capacitor exists between the other surface of the proof mass and the other electrode.
Electrically conductive lead wires are attached to each of the three conductive silicon layers and are fed to known DC or low frequency AC electronics, e.g., operational amplifier configurations. The electronics provide an output electrical signal indicative of the change in capacitance caused by displacement of the proof mass about the hinges toward either of the two metal electrodes due to acceleration applied to the sensing element.
Also, one of the outer silicon layers is typically treated as a base with which the sensing element is mounted to a rigid board, e.g., a ceramic circuit board. The large flat plane of the board is then placed at right angles to the acceleration to be sensed.
The sensing element cannot be directly rigidly mounted to a typical ceramic circuit board because the difference in thermal expansion coefficients between the circuit board and the silicon base of the sensing element causes uneven external stresses to be exerted on the sensing element over the operational temperature of the device. Such stresses cause the proof mass and the gaps between the proof mass and the electrodes, to become offset and give erroneous acceleration measurements. Also, such stresses weaken and deteriorate the solder joint attachments or connections between the sensing element and the board. As a result, the sensing element is typically mounted to the circuit board with spacers, e.g., one or more tiny spherical balls at each contact point, coated with an adhesive (e.g., silicone rubber) which is compliant over temperature to relieve any thermal expansion mismatch between the sensing element and the board. Typically, flying (or dangling) electrical leads (or wires) extend from the sensing element to such circuit board. Also, the board to which the sensing element is mounted is typically mounted by right angle brackets to a separate circuit board which contains the electronics to interface with the sensing element. Such a bracket allows the electronic circuit board to be housed in a horizontal position while the sensing element is responsive to horizontal acceleration, e.g., for an automotive airbag application.
The attachment of such flying leads and the attachment and precision orientation of such angle brackets require extra steps in the manufacturing process. Such a mounting technique makes such sensors difficult and costly to manufacture. Also, use of such thermally compliant adhesive is difficult to work with when manufacturing microelectronics due to potential for contamination of the electronics.
Thus it would be desirable to eliminate such deficiencies of the prior art techniques.